1. Field of the Invention
This invention broadly relates to a process for the removal and subsequent conversion of sulfur compounds found in hydrocarbons, especially refractory sulfur compounds found in petroleum streams (blend stocks) used to make gasoline and diesel fuel, to useful oxygenated hydrocarbon products and sulfur dioxide. Thus, the invention broadly relates to producing a hydrocarbon stream of a lowered sulfur content. The invention particularly relates to a catalytic gas phase oxidation process using a supported metal oxide catalyst or a bulk metal oxide catalyst for treating refractory sulfur compounds found in petroleum streams, such as used to make gasoline and diesel fuel, and converting them to useful oxygenated hydrocarbon products, such as maleic acid (anhydride), phenol, benzyl aldehyde and benzoic acid, and to sulfur dioxide. This method can also be extended to the removal of sulfur typically present in such sulfur containing hydrocarbons as gasoline and diesel fuels to yield sulfur dioxide and sulfur-deficient hydrocarbons.
2. Description of Related Art
Diverse types of petroleum feedstocks and streams contain sulfur compounds whose removal often is indispensable for commercial utilization of the feedstock or stream and/or for subsequent processing of the feedstock. In the face of ever-tightening sulfur specifications in transportation fuels, such as gasoline and diesel fuel, sulfur removal from petroleum feedstocks used to makes such fuels and from the petroleum fuel products themselves will become increasingly more important in years to come. In this regard, there have been several studies by the EPA concluding that the presence of sulfur in gasoline has an adverse impact on catalytic converters and, thus, tailpipe emissions from automobiles.
Sulfur deactivates conventional three-way Pt/Pd/Rh/Al2O3 catalytic converters designed to reduce hydrocarbon, CO and NOx emissions. Sulfur also degrades automobile diagnostic systems. Gasoline sulfur also prevents the introduction of more advanced catalytic technologies, such as Pt/BaO-based catalysts as NOx traps. For diesel fuels, the presence of sulfur produces an additional problem since particulate emissions created during combustion are increased in the presence of sulfur. There is special concern for particulates less than 2.5 microns since the EPA has concluded that there is a stronger link than ever between the tiniest soot particles and thousands of premature deaths each year. Consequently, both the EPA and the DOE have recommended that significantly limiting the level of sulfur in gasoline (15 ppm) and diesel fuels (30 ppm) would be essential for meeting lower vehicle emission standards in the future (by 2007). It is no surprise that substantial efforts have been expended to eliminate sulfur compounds from petroleum products.
Sulfur compounds routinely found in petroleum feedstocks and products include thiols (RSH), sulfides (RSR), disulfides (RSSR), saturated cyclic sulfides (C2-C5-cyclic sulfur-compounds, which incorporate sulfur into the saturated ring structure), thiophenes (primarily unsaturated C4-cyclic sulfur compounds, where sulfur is incorporated into the unsaturated ring structure) and thiophene derivatives such as benzothiophene and dibenzothiophene (benzene rings that are fused to the sides of the thiophene (unsaturated C4-cyclic sulfur compound) and various substituted benzothiophenes and dibenzothiophenes. Sulfur is the most abundant heteroatom impurity in petroleum crude and varies from 0.1 to 5 wt % depending on the geographic origin of the petroleum. After distillation of the crude oil, the sulfur content increases with the fraction's boiling point: naphtha (0.01-0.05% sulfur), kerosene (0.1-0.3% sulfur), gas oil (0.5-1.5% sulfur), atmospheric residue (2.5-5% sulfur), vacuum gas oil (1.5-3% sulfur), and vacuum residue (3-6% sulfur).
For the low boiling naphtha fraction, sulfur is mainly present as thiols, sulfides, disulfides or thiophene. For the middle boiling kerosene and gas oil fractions and especially the higher boiling fractions, thiophenic compounds, particularly benzothiophenes, dominate.
As a general rule, simple aliphatic, naphthenic, and aromatic mercaptans, sulfides, di- and polysulfides and the like surrender their sulfur more readily than the class of heterocyclic sulfur compounds comprised of thiophene and its higher homologs and analogs. Within the generic thiophenic class, desulfurization reactivity decreases with increasing molecular structure and complexity. While simple thiophenes represent the more labile sulfur types, the other extreme, sometimes referred to as “hard sulfur” or “refractory sulfur,” is represented by the derivatives of benzothiophene and dibenzothiophene, especially those mono- and di-substituted and condensed ring dibenzothiophenes bearing substituents on the carbons beta to the sulfur atom. These highly refractory sulfur heterocycles resist desulfurization.
Conventional technology removes sulfur from petroleum feedstocks via catalytic hydrodesulfurization (HDS). Hydrodesulfurization is one of the fundamental processes of the refining and petrochemical industries. In HDS, sulfur removal is typically achieved by reaction of the sulfur compounds with hydrogen over non-noble metal sulfides, especially those of Co/Mo/Al2O3 and Ni/Mo/Al2O3 catalysts, operating at elevated temperatures (˜400° C.) and extremely high pressures (˜100 atmospheres). Under these somewhat severe reaction conditions, (1) RSH, RSR and RSSR react to form hydrocarbons (RH), (2) saturated cyclic sulfides are converted to alkanes, (3) thiophene reacts largely to mixed isomers of butene (C4H8), (4) benzothiophene and its derivatives are initially hydrogenated to thiophane derivatives before removal of the sulfur atom to finally yield ethylbenzene (Bz-CH2CH3) and (5) dibenzothiophene is mainly converted to biphenyl with small amounts of phenylcyclohexene. The sulfur itself, along with the H2, is ultimately converted to hydrogen sulfide. This H2S is subsequently reacted with O2 in the Claus process to H2O and elemental sulfur, which is disposed in special landfills. An overall hydrogen balance for the HDS process reveals that the very valuable and expensive H2 ultimately gets converted to invaluable H2O.
Hydrogen consumption, thus, is an important consideration in these hydrodesulfurization (HDS) reactions because many of the components present in the feedstocks are more valuable as unsaturates, especially aromatics and olefins, and the hydrogenolysis of such components results in the production of light gases with marginal fuel values. In addition, during the manufacture of H2 a significant amount of global warming CO2 is generated during the very energy intensive steam reforming of methane, or steam reforming of lower hydrocarbons. The H2 is typically generated by steam reforming of CH4, or lower hydrocarbons, and the water-gas shift reaction as follows:CH4+H2O⇄CO+3H2CO+H2O⇄CO2+H2.
Thus, the current HDS process technology converts valuable H2 to invaluable H2O, reduces the octane of the gasoline feedstocks, generates global warming CO2 and elemental sulfur that needs to be disposed and is extremely energy intensive.
Notwithstanding these drawbacks, the petroleum industry has stated that HDS will be the preferred approach they will use to reduce sulfur levels in response to tighter regulatory controls because, HDS is a well-established and proven technology.
While HDS, as currently practiced, is known to provide nearly complete removal of mercaptans, sulfides and disulfides from liquid hydrocarbons, use of the current designs for reducing the level of thiophenes and other refractory sulfur compounds to a level of 30 ppm or below is problematic. In order to meet this very low level of sulfur, petroleum refiners will have to build additional capacity for generating additional hydrogen and will have to increase the reactor capacities of their HDS units or develop significantly more active HDS catalysts. Furthermore, efforts to drive the current HDS processes to increased sulfur removal is likely to lead to increased hydrogenation of the valuable fuel components and degradation in the fuel value (octane reduction) of the treated petroleum feedstock.
While HDS remains the predominant commercial approach for desulfurizing petroleum products, particularly petroleum feedstocks for making gasoline, the prior art has continued to develop and examine alternative processes. For example, various oxidative processes are known for removal of mercaptans by converting them to disulfides; such as the Merox™ process (see Handbook of Petroleum Refining Processes, R. A. Meyers, editor-in-chief, chapter 9.1, McGraw-Hill Book Company (1986)). It is also known to remove mercaptans and disulfides from petroleum feedstocks by adsorption with clays. U.S. Pat. No. 5,360,536 uses an adsorbent of a solid solution of metal oxides.
U.S. Pat. No. 5,935,422 describes a process for removal of organic sulfur compounds, particularly heterocyclic sulfur compounds, from petroleum feedstocks, and especially FCC feedstocks, by adsorption using zeolite Y exchanged with an alkali or alkaline earth metal cation and preferably impregnated with a group VIII metal. Regeneration of the sorbent is achieved by beating the sulfur-laden adsorbent in a hydrogen atmosphere.
The prior art also is exploring the use of biological removal processes (biodesulfurization). For example, U.S. Pat. No. 6,130,081 relates to a method of degrading organic sulfur compounds such as benzothiophene, dibenzothiophene and the like, by use of microorganisms belonging to the genus Paenibacillus and having the ability to decompose organic sulfur compounds, especially heterocyclic sulfur compounds, by specifically cleaving their C—S bonds under elevated temperature conditions.
Te et al., “Oxidative reactivities of dibenzothiophenes in polyoxometalate/H2O2 and formic acid/H2O2 systems,” Applied Catalysts A: General, 219(2001) 267-280 describes a liquid phase oxidation process potentially useful for removing refractory sulfur compounds from liquid hydrocarbon feed steams. The sulfur compounds are oxidized to sulfones and sulfoxides, which then can be extracted from the hydrocarbon. Other liquid phase oxidative approaches are described in EP 565 324 and U.S. Pat. No. 5,910,440 (biocatalytic).
I. G. Fedorchenko, N. N. Nechiporenko, V. I. Mitryaeva and E. N. Dubranovskaya, “Catalytic Activity of Certain Metal Oxides in Oxidation of Sulfur Compounds,” Vesten. Khar'kov. Politekh. Inst. 13 (1966):44-47 describes work involving the oxidation of thiophene over active metal oxides mixed with pumice, 1/4 ratio. Best results were obtained with Fe2O3, MoO3 and Al2O3, but the reaction products obtained under the chosen reaction conditions were COx, SO2 and H2O. A similar combustion study of thiophene was reported by V. A. Sslavinskaaya, D. Kreile, D. Eglite and I. Geimane, “Formation of Carbon Monoxide and Carbon Dioxide in the Vapor-Phase Oxidation of Heterocyclic Compounds on Vanadium-Molybdenum-Phosphorous Catalysts,” Latv. PSR Zinat. Akad. Vestis, Kim. Ser. 6 (1971): 735-738. These investigators employed a V—Mo—P catalyst.
Another interesting investigation of thiophene oxidation over a 10% MoO3 and TiO2 mixed metal oxide catalyst is reported by M. Blanchard and J. Goichon, “Heterogeneous Catalytic Oxidation of Aromatic Sulfur Compounds: Thiophene and Benzothiophene,” Bull. Soc. Chim. Fr. 1-2/Pt. 2 (1975): 289-290. The oxidation yielded 75% selectivity towards maleic anhydride and thiomaleic anhydride at moderate conversions. Supported MoO3/TiO2 and bulk V2O5 were also found to be efficient catalysts for the selective oxidation of thiophene to maleic products. In addition, oxidation of benzothiophene over the 10% MoO3—TiO2 catalyst quantitatively yielded phenol with 100% selectivity.
U.S. Pat. No. 5,969,191 describes a catalytic thermochemical process, which can be used for converting by-products from pulp and paper mills (TRS compounds including mercaptans) to a valuable chemical intermediate (H2CO), which is consumed in the pulp and paper Industry. A key catalytic reaction step in the thermochemical process scheme is the selective catalytic oxidation of organosulfur compounds (e.g., CH3SH+2O2→H2CO+SO2+H2O) over certain supported (mono-layered) metal oxide catalysts. The preferred commercial catalyst employed in this process consists of a specially engineered V2O5/TiO2 catalyst that minimizes the adverse effects of heat and mass transfer limitations that can result in the over oxidation of the desired H2CO to COx and H2O.